This PDF file contains the front matter associated with SPIE Proceedings Volume 11834, including the Title Page, Copyright information, and Table of Contents.

Experiments were conducted at the TISTEF laser range to evaluate the atmospheric turbulence data of several instruments. A Scintec BLS900, BLS2000, SLS20, MZA DELTA, and an Applied Technologies SATI-3A sonic anemometer were deployed on the 1 kilometer range and recorded measurements over a multi-day period. The data was then processed to compare the calculated refractive index structure parameter (C²_n ) between the instruments. The BLS2000 and DELTA were also deployed to record turbulence measurements along a 13.5 kilometer slant path from the TISTEF site to the Vehicle Assembly Building roof on Kennedy Space Center property for additional evaluation.

This work presents an extended analysis of atmospheric refraction effects captured by time-lapse imagery for near-ground and near-horizontal paths. Monthly trends and multipath analysis of image shift caused by refraction during daytime are studied. Nighttime shift measurements during moonlit nights are also presented. Advanced nonlinear machine learning approaches for image shift prediction are implemented and the performance of the models is evaluated.

Measurement of atmospheric turbulence has long been carried out using differential temperature sensors, DTS. DTS measurements of small temperature fluctuation differences between two spatially separated thermocouples can be ensemble averaged to calculate the temperature structure constant which can be used to approximate the refractive index structure constant. This technique has been widely applied and used with reasonable success in spite of several important limitations. One of the primary limitations is that the temperature structure constant alone does not fully define the refractive index structure constant; because the water vapor structure and water vapor to temperature cross correlation terms, which are much more difficult to measure, are ignored. This paper presents an initial feasibility analysis and experimental validation for a new type of refractive index structure constant sensor that is based on a compact, low cost interferometric sensor. This method is being explored as a potential alternative to DTS, because it measures index of refraction directly and thus offers the possibility of significant improvement in measurement accuracy. The paper outlines the sensor concept, its key components, and an analytical and experimental validation of the refractive index structure measurement accuracy.

Optical propagation through deep turbulence is important to understand for applications such as free space optical communication The United States Naval Research Laboratory has maintained a 16 km range across the Chesapeake Bay for over a decade. A 1550 nm beam is transmitted from one end to the other where a variety of instruments measure optical parameters such as transmission, scintillation index, aperture averaging, and angle of arrival variation. In addition, meteorological parameters are collected using a weather station at CBD as well as NOAA buoys on the bay. In this work, we present comparisons of measurement optical scintillation index, distribution functions and aperture averaging to calculations using extended Rytov theory and and wave optics calculations.

We present and analyze an experiment to characterize the effect of turbidity to pulsed optical time transfer through water. During optical time transfer through water, two modules are each equipped with an atomic clock and timing electronics to keep and compare time independently. One module emits a laser pulse to a remote clock. The departure of the laser pulse from is time stamped with respect to a reference clock. The arrival of the pulse at the remote unit is time stamped with respect to its clock. The discrepancy between clocks is computed by comparing the measured departure time from plus the estimated time of flight of the pulse to the measured time of arrival at the remote unit. The estimated time of flight and the resulting time transfer performance are affected by the pulse propagation channel conditions. To analyze the effects of various channel conditions, we present the clock discrepancy, the measurement noise of an array of time stamps, and the Allan deviation. Allan deviation is a common metric in the time and frequency community to evaluate the stability of a series of events, where lower values are correlated with greater stability. This is an especially effective tool for studying nonstationary processes, where mean and deviation are a function of time, rather than constant. Additionally, insight into the dominant noise contributions of the frequency instability can be drawn by plotting the Allan deviation versus averaging time. In order to evaluate the free-space optical time transfer, Allan Deviation plots are generated for the empty, still water, and turbid water filled tank. Optical time transfer Allan deviation is compared to Allan Deviation generated from simultaneous frequency counter measurements through coaxial cables to differentiate between clock and channel stability.

During a NASA Phase II SBIR project the Goodman Technologies (GT) team developed and competed two advanced processes for producing large silicon carbide (SiC) mirror substrates and structures, both processes which could ultimately be performed in the microgravity environment of space. The process of scale-up from Phase I mirror substrates was anything but easy; over 250 unsuccessful trials were performed before achieving a printable substrate. A new Z-process allowed for conversion of moldable (but not printable) nanopastes into Pathfinder mirror components. The team also overcame delays associated with shutdowns due to COVID-19. The work did ultimately result in the demonstration of the world’s large 3D/AM SiC mirror substrates (25-cm scale). The first process, the Robocasting or Direct Ink Writing (DIW) printing process, was employed to 3D print engineered nanopastes consisting of SiC particulates with sizes ranging from the nanometer to micron scales mixed and suspended homogeneously in a preceramic polymer and liquid solvent solution. A large computer numerically controlled (CNC) platform with a 1.2-meter by 1.2-meter build bed was modified to become a large prototype “robocaster”. Modifications included incorporation of a large build plate (an optical bench top), a fluid supply system and syringes, a 750 W infrared (IR) heater, and programming. More than 200 experiments were conducted on the prototype robocaster, unsuccessfully, over a span of 16-months. Every print job would crack, and/or warp, and/or delaminate either during printing, or during low temperature curing, or during low-temperature polymer infiltration pyrolysis (PIP), or during high-temperature (PIP). Through massive effort, we finally overcame the engineering issues. Using a production robocaster, the ability to 3D print, join and then cure individual off-axis parabolic (OAP) mirror substrate segments (4 of them) to make a 25-cm monolithic Pathfinder mirror substrate for subsequent densification and pyrolysis was ultimately demonstrated. Two 25- cm monolithic substrates were printed and cured successfully. All of the robocast parts ultimately warped or bowed and/or cracked during low-temperature pyrolysis or high-temperature PIP steps. Through internally funded efforts performed by GT it was confirmed that as cured robocast material can be silicon melt infiltrated to form a very low silicon content of reaction bonded silicon carbide. GT also found that robocast material densified via polymer infiltration pyrolysis results in a polishable material. The second process GT developed and employed with UHM is the Z-process. The Z-process starts with a moldable (but not printable) nanopaste consisting of SiC particulates with sizes ranging from the nanometer to micron scales mixed and suspended homogeneously in a preceramic polymer and liquid solvent solution. The moldable nanopaste is compacted in a custom designed metallic and graphitic tooling set which also contains a precision mandrel. Compaction serves to squeeze out “extra” liquid phase material from the nanopaste while conforming the nanopaste to the shape of the mandrel prior to curing. Once cured the part is almost theoretical density and requires only a few steps of PIP to fully densify the part. PIP is accomplished at temperatures much lower than silicon melt infiltration (<1600 °C), chemical vapor deposition (<1450 °C), or conventional sintering (<2200 °C), providing enormous energy savings. The Z-process tooling can be used multiple times providing economy of numbers. The Z-process was successfully used to join 4-OAP segments and a backside lattice supporting structure. This paper shall discuss the results of progress made towards the manufacturing of Large SiC Space Optics traceable to meter-class segments for a far-infrared surveyor (the Origins Space Telescope, OST). The technology also is intended to fill priority technology gaps for the LUVOIR Surveyor, and Habitable Exoplanet Observatory (HabEx).

By enabling Free Space Optics (FSO) technology as complementary solution to RF systems, the next generation satellite communication that relies on optical links is on the verge. Even though the transition to wireless optical communication is a fact, the space domain is very conservative to such critical changes that call for close evaluation of each system aspect. Since trade-off between costs and efficiency is required, a state-of-the-art laboratory testbed for verification of satellite-to-ground APD-based (Avalanche Photodiode) FSO links subject to atmospheric turbulence-induced fading is proposed in the current paper. In particular, the self-developed hardware channel emulator represents an FSO channel by means of fiber-coupled Variable Optical Attenuator (VOA) controlled by driver board and software. Having addressed real atmospheric Radiosonde Observation (RAOB) databases for Vienna, Austria, highly precise optical attenuation data due to atmospheric turbulence fading are generated and applied into the considered software. The used approach relies on complex analysis simulating atmospheric vertical profile of refractive index structure parameter as well as Gamma- Gamma and Log-Normal scintillation models considering both parameters the telescope aperture and the elevation angle. Along with the FSO channel emulator, the receiver under-test is high-speed 10 Gbps APD photodetector with integrated Transimpedance Amplifier (TIA) that is typically installed in future OGSs (Optical Ground Stations) for LEO/GEO satellite communication. Having considered On-Off Keying (OOK) Intensity Modulation/Direct Detection architecture, the emulated optical downlink is evaluated based on two different data throughputs while atmospheric turbulence induced-scintillations are also taken into account. The overall testbed performance is addressed by a BER tester and a digital oscilloscope, providing high-quality BER graphs and eye diagrams that prove the applied approach for testing APD-TIA in the presence of scintillations. Furthermore, the accuracy of the hardware channel emulator is evaluated by means of calibration measurements as well as beam camera providing measured proof of the propagated high-quality laser beam.

We propose and demonstrate a communication system based on compact optics, using Raspberry Pi and a FPGA Basys3 boards at a speed of 50 Mbps through a propagation distance of 500 meters in medium to strong turbulence. Results in various scenarios and remaining challenges are presented.

A key component of establishing free-space optical communication between ground stations, satellites, and lunar nodes consists of accurate pointing, tracking, and beam acquisition. This investigation explores the use of sensor arrays external to the receiver aperture for sampling the incoming transmitted beam for the purpose of estimating the pointing error. Different arrays of sensors and different combinations of sensors processed with the traditional quadrant detector algorithm were evaluated based on the ability to accurately predict the direction of pointing error under a variety of beam widths and turbulence conditions. MATLAB simulations demonstrate that different configurations perform best under different conditions, and thus a combination of configurations with flexible processing capabilities provides the most promising approach. Beam width proved to be the most important factor in prediction accuracy.

The paper will present an overview of atmospheric turbulence, turbulence limited imaging, and a Digital Adaptive Optics system that mitigates atmospheric turbulence in passive imaging systems.

The atmospheric propagation of Near Infrared (NIR) high-power laser beams is affected by the thermal interactions of the electromagnetic beam with the air and the phase perturbations caused by the air inhomogeneities. These interactions lead to inefficient delivery of energy to far field surface. In this work, the implementation of a simulation model integrating thermal distortions induced by the laser beam and the turbulence of atmosphere is presented. Additionally, an iterative learning method is integrated in the simulator to correct the laser beam profile using a deformable mirror. Simulations are realized for a 1.5 kW laser beam at 1064 nm propagating along 150 m propagation path.

We present several solutions to problems particular to adaptive optics for free-space laser-based communications. Specifically, for scenarios where strong scintillation is present, we have developed a digital, adaptable Shack-Hartmann wavefront sensor, as well as the modal holographic wavefront sensor based on the Karhunen-Loève modes. Additionally, using the same modal basis and optimization algorithms from deep learning, we have improved upon stochastic parallel gradient descent wavefront-sensorless approach. For underwater communications, we have set up a water tank and demonstrated real-time adaptive optics in the visible. For deep-space downlinks, we have investigated several wavefrontsensing modalities with respect to their robustness to very low signal-to-background ratios expected during daytime. We also present results of data transmission experiments using coherent modulation over a 400-m double-pass horizontal link.

The increase of data rate and bandwidth efficiency of free-space optical communication links may be supported by the use of dense orbital angular momentum (OAM) states, carrying several information bits per transmission. Using machine-learning decoding, the performance of 32-OAM and 64-OAM signal constellations –designed using 4-state superpositions– are studied using numerical propagation models. Using two candidate architectures for detection –Shack-Hartmann and Mode Sorter– we evaluate the performance of the modulation in a simulated optical atmospheric channel by means of the detection accuracy.

The analysis of superpositions of Orbital Angular Momentum (OAM) modes is a challenging problem, particularly when atmospheric turbulence is present or when the phase structure of the wavefront is not available. In such conditions it is not possible to correct the distortions and reconstruct the vorticial phase structure: the rings and petals that characterize the intensity profiles of such beams become deformed and may even lose integrity. These artifacts may compromise the possibility of establishing free-space optical links based on OAM superpositions. We propose using a particular selection of Laguerre-Gauss modes and convolutional neural networks for a reliable classification of superpositions of two modes. The network (based on a pre-trained network AlexNet that combines convolutional and fully-connected layers) is trained as a classifier based on 2-d intensity profiles that can be obtained from a digital camera. For illustrating the proposed method, we used simulations of light beams propagated through L = 1 km with three levels of turbulence: C²_n ∈ {2×10^-15, 9.24×10^-15, 2.9×10^-14} m^-2/3. The emitted beams are made up of 2 different Laguerre-Gauss modes with OAM between -15 and +15, and radial indices between 0 and 3. Classification results show that the radial index can be used effectively to enlarge the set of information symbols.

When propagated through atmospheric turbulence, Orbital Angular Momentum (OAM) modes suffer a loss of orthogonality that can compromise their detection and classification. The problem is more challenging when user information encoded on multi-state OAM superpositions needs to be detected with high probability. Optical sensors like the Shack-Hartmann detector or the Mode Sorter are candidates for such task. We describe how OAM histograms derived from such detectors can be used for decoding the original data symbols. We propose Machine Learning strategies for a reliable classification of the histogram patterns obtained with 4-mode superpositions propagated over a 1 km range in weak to intermediate turbulence.

Propagation of laser beams through a turbulent atmosphere over extended ranges can cause significant beam scintillation and wander which can degrade the effectiveness of a Free Space Optical (FSO) link. The use of a spectrally broadband laser light source, with a high spatial coherence and short temporal coherence, could lead to improved performance in one or both of these areas. This experiment investigates the effect of temporal coherence on the far-field turbulence induced effects on the beam. Narrow linewidth coherent sources were compared against a broadband source over a 13.5 km slant-path. The path was instrumented with a path averaged turbulence monitoring device during data collection along with a range of other meteorological parameters to predict atmospheric parameters. Target board beam profile data was collected to measure the spatial statistics due to atmospheric turbulence along with silicon detectors to measure the temporal statistics of the atmospheric turbulence effects. This data is analyzed and compared to full diffraction wave propagation simulation results. Our analysis shows the benefit that the broadband source does not suffer as many scintillation effects as the narrow-linewidth sources.

Structured laser beam propagation through inhomogeneous media is of interest for remote sensing and spatial division multiplexed free-space optical communications. The structured light of interest contains orbital angular momentum (OAM) that inherently forms an orthogonal basis set. The decomposition of an optical wavefront into OAM modes is analogous to an azimuthal Fourier transform that provides angular information. When a wavefront propagates through an imperfect optical system or an inhomogeneous medium, the inhomogeneities manifest themselves as phase distortions along the optical wavefront. This study aims to understand how phaseonly distortions shape a beam’s OAM spectrum. The azimuthal information of the distortions will be explored using the Fourier duality between the angular position and azimuthal frequency. To present this perspective, the example of an azimuthal aperture will be used to display the angular uncertainty principle. This concept will be further discussed using an example of a phase-only distortion represented by a common lens aberration using Zernike polynomials and then to the more complicated scenario of a random medium. It is found that the Fourier duality can be used to calculate the OAM spectrum of a random phase distortion. For the case of a finite beam incident on the distortion, it is found that the beam size and spatial structure play a role in spreading the beam’s OAM spectrum. It is seen that a beam’s OAM spectrum spread is independent of the mode if the beam size is taken into account.

In recent years the study of structured light, specifically orbital angular momentum (OAM), for remote sensing and free-space optical communications has gained great interest. Laser beams propagating through the atmosphere are susceptible to optical turbulence, which leads to beam distortions in the form of scintillation, beam wander, increased beam spreading, and loss of temporal and spatial coherence. One method that has been predicted to reduce scintillation is the use of partially coherent beams that can be propagated via their coherent mode representation (CMR). For the case of communications this is of great importance as scintillation reduction can lead to improved link effectiveness. For a spatially division multiplexed system employing OAM, reduction of the beam’s spatial coherence will possibly impede on the ability to separate the spatial modes on the output. A partially coherent beam carrying OAM through its CMR is the I_m Bessel beam that recently has been experimentally realized. To expand upon the possible uses of the I_m-Bessel beam, this paper intends to take a first step at simulating and quantifying the beam’s CMR through time-correlated optical turbulence. In this way, traditional beam metrics such as scintillation, beam spreading, spatial coherence, and OAM spectrum can be quantified. New degrees of freedom, such as CMR cycling rate and detector sampling rate relative to the turbulence, will also be explored.

Probability density functions of Gaussian laser beam irradiance measured after propagation over 7km atmospheric path in comparison with various theoretical models are presented. The initial laser beam diameter at the e^-2 intensity level was about 6 mm, the receiving aperture size was 14.4 cm. The experimental observations were performed in a wide range of turbulence strengths. The cases of weak, moderate, and strong intensity fluctuation regimes have been analyzed. Different receiving aperture radiuses were considered. The chi-square metric was used to estimate the agreement between the experimental and different theoretical statistics. The fractional gamma distribution has shown the best results for probability density distributions of apertures with sizes about 1 cm and 4 cm under strong turbulence conditions. The aperture averaging effect results in excluding near-zero irradiance values, which are typically observed on-axis at strong turbulence and high value of scintillation index which qualitatively transforms the observed statistics, so that experimental probability density functions can be well approximated by the fractional gamma distribution. With the increase of the aperture size, a further transformation of the statistics was observed. The statistics of experimental data for moderate and weak fluctuation regimes approached the lognormal and gamma distributions.

Laser propagation through a medium is often accurately explained by geometrical optics where light is described as rays. When considering wave diffraction, a more generalized description of physical optics is used. Faced with the complexity of interactions of a laser beam with the atmosphere, and the additional challenges of deploying a physical high power laser system for advanced evaluation in harsh environment, the use of simulation software is preferred to study and predict the behavior of the beam propagation in atmosphere. In this paper, we show that the intensity and diffraction of a laser beam in the atmosphere can be properly described by geometrical optics when coupled with an appropriate model of the variations of the index of refraction, heat convection velocity, and atmospheric relative humidity. The study uses a Near Infrared (NIR) laser source at 980 nm of 1 kW average power. The laser beam propagation was implemented using a finite element model developed with a COMSOL multi-physics ray tracing model in both transient and steady-state regimes. The beam divergence from the center of the propagation path was clearly observed when the crosswind velocity inside the domain was greater than 10 m/s while having 300K temperature at 1 atm pressure as initial conditions. The direction and amount of divergence are observed to be directly linked to the velocity of the cross-wind, as well as the refractive index variations due to the amount of humidity in the air, and the heat generated by the laser beam in the atmosphere. According to our results, the higher the humidity of the air is, the more energy is deposited in the atmosphere resulting in the reduction of the accumulated power on the target.

Split-step wave-optical simulations are useful for studying optical propagation through random media like at- mospheric turbulence. The standard method involves alternating steps of paraxial vacuum propagation and turbulent phase accumulation. We present a semi-analytic approach to evaluating the Fresnel diffraction integral with a phase screen in the source plane, which is a key building block in split-step simulations. Compared to the standard angular-spectrum approach using the fast Fourier transform, the semi-analytic method provides relaxed sampling constraints and an arbitrary computational grid. Also, when a limited number of observation-plane points are evaluated or when many time steps are used, the semi-analytic method computes faster than the angular-spectrum method.

The performance of free space optical applications depends on accurate estimation of the trajectory of optical beams through the atmosphere. In situations where a signal at the wavelength of interest is not available at the target, the propagation path of an optical beam may be predicted based on the refractive index gradient profile of the atmosphere, typically using standard models such as the 1976 US Standard Atmosphere. However, the actual refractive conditions evolve with time and the formation of features, such as inverse temperature layers and ducts, can introduce strong refractive index gradients. We present ray tracing studies involving modeling and measurements of the effects of near-ground atmospheric refraction on near-horizontal beam propagation during daytime along a desert path at the Jornada Experimental Range in Las Cruces, NM. The amplitude of the diurnal deviation of the ray trajectory of a 1550-nm source is observed in simulation results where the refractivity profile was generated from numerical weather prediction. Visible time-lapse camera measurements of diurnal differential image effects (compression/stretch) are also compared with results predicted by numerical weather modeling. Additionally, a duct-like refractivity profile occurring in the morning at the site and whose parameters are estimated from time-lapse imagery, is imposed on the US standard atmosphere and the resulting differential trajectory effects are demonstrated.

In this work, a set of coupled equations is presented that describes the spot size and pulse length evolution of a Kerr focused ultra-short laser pulse in a turbulent and group velocity dispersive atmosphere. Solutions to the equations are compared against Monte Carlo simulations for focused and collimated beams in weak, moderate, and strong turbulence. The results indicate good agreement except when the beam accumulates excessive wings in the transverse profile and/or undergoes pulse splitting, such that the self-similar evolution assumption in the coupled equation derivation breaks down.

We report on our progress in using light-fields of a distant, static object, observed through distributed volume turbulence to recover an estimate of the turbulence phase at discrete intervals along the path. Our approach is similar to those used by phase diversity and multi-conjugate adaptive optics but instead uses imagery from a single light field frame. Similar to those approaches ours provides a joint estimate of both the scene and the turbulence volume. We present results featuring multiple scenes, integrated turbulence strengths, and isoplanatic angles. Light field sampling effects are also considered.

Accurate characterization of atmospheric refractive index fluctuations (optical turbulence) is important in applications such as Free-Space Optical (FSO) communication and laser remote sensing, where atmospheric optical propagation is involved. In this paper, we present the statistics of near-surface optical turbulence derived from three-year sonic anemometer-thermometer observations at a semi-arid, flat terrain in peninsular India. Using concurrent and collocated measurements of meteorological fields and atmospheric aerosols (both scattering and absorbing type), the role of atmospheric boundary layer dynamics and aerosols in modulating the magnitude, evolution, and temporal variations (over diurnal and seasonal scales) of refractive index structure parameter (C_n²) are discussed. Absorption of solar radiation and the resulting atmospheric heating by aerosol particles will modify the land-atmosphere temperature gradient that regulates the near-surface C_n². We discuss such a scenario using aerosol black carbon measurements close to the surface of the Earth and highlight the crucial role of ABL dynamics in controlling the influence of such aerosol radiative heating effects on C_n². These results will be helpful in improving the weather model simulations of optical turbulence over semi-arid regions.

Long-range imagery systems are negatively affected by atmospheric turbulence. This paper describes a method for quantifying that turbulence strength profile along the viewed path by presenting a novel system concept and first measurement results.

Atmospheric turbulence parameters are measured routinely across the Chesapeake Bay. The 16.2km propagation path is slightly sloped, with the laser transmitter at a height of 30m while the receivers are at 5m above sea level. We measure angle-of-arrival variance to give C_n² and irradiance variance over various apertured receivers to give σ_I².The Navy Atmospheric Vertical Surface Layer Model (NAVSLaM) is used to predict C_n² that is based upon prevailing meteorological parameters. Comparison with measured C_n² over extended time periods shows reasonable correlation. A value of the inner scale of turbulence l0 is also predicted from NAVSLaM. This paper looks at the significance of using C_n² and l₀ from NAVSLaM in standard analytical models of scintillation.

Wavefront distortion of data-carrying laser beam propagating through the atmosphere has been reported to have detrimental effects on the performance of Free-Space Optical (FSO) communication systems. Optical intensity fluctuation models generally assume clear air optical turbulence where atmospheric aerosols warming effects are neglected. This variation of the refractive index structure parameter (C_n²) of the atmosphere due to the aerosol induced warming and its influence on Bit Error Rate (BER) performance of FSO systems are studied in this paper using high-resolution radiosonde and multi-satellite observations of aerosols and atmospheric thermodynamics. Based on an approximate mathematical expression built on Gauss-Laguerre quadrature rule, and a radiative transfer model-based analysis, the BER of a Differential Phase Shift Keying (DPSK) FSO communication link through Exponentiated Weibull modelled turbulence with aperture averaging has been investigated. Our results show significant signal deterioration with the aerosol-induced turbulence taking a toll on the signal to noise ratio (SNR) over more than 15 dB. BER analysis under different receiver aperture dimensions is performed with the selected intensity fluctuation model. We show that aperture averaging does not have significant influence on the performance enhancement under aerosol perturbed atmospheric conditions.